This invention relates to an inductive angle sensor for a motor vehicle having an oscillator circuit that generates a periodic AC signal in an exciting coil, and having at least one receiving coil and an evaluation circuit for detecting a signal induced in the receiving coil, or coils, and a movable inductive coupling element that influences a strength of inductive coupling between the exciting coil and the receiving coil(s).
European patent application (0 159 191 A2), for example, discloses such an angle sensor in an embodiment of an inductive position sensor shown in FIG. 3 thereof. Inductive position sensors, particularly angle sensors, are being used increasingly, particularly in motor vehicles for replacing resistance sensors (potentiometers) which are still widely used today, because, among their other advantages, they operate without contact and are thus practically wear-free and they can also be structured so that they are largely insensitive to interferences caused by soiling or dirt contamination.
A general functioning principle of inductive position sensors is based on an AC signal being applied to an exciting coil which, in turn, induces AC signals in one, or preferably in a plurality of, receiving coils. An inductive coupling between the exciting and receiving coil(s) depends on positions of two elements of the sensor that move relative to each other; specifically on either the relative positions of exciting and receiving coils that move relative to each other or on a position of an inductive coupling element (structured as a metallic or magnetic object or a short circuit line) that moves relative to the exciting and receiving coils, with the inductive coupling changing as these positions change.
To determine a position or an angle of rotation, an evaluation circuit is used that detects amplitude values and/or phase relationships of receiving coil signals.
One particular problem with inductive sensors, especially those used in motor vehicles, is that they are sensitive to electrical, magnetic, and electromagnetic fields, which can falsify signals picked up by the receiving coil(s).
This is especially a problem in motor vehicle applications, because in this setting a number of interference sources can be present in a very narrow space and, additionally, depending on an application, an operational reliability of a sensor is certainly safety related, such as when the inductive sensor monitors an angular position of an accelerator pedal or of a throttle valve.
Electromagnetic interference can be controlled as a rule by suitable filtering (e. g. by a bandpass filter). However, this is difficult if an interference frequency is close to an operating frequency of the oscillator and an appropriately steep filter necessary to filter out these interference frequencies cannot be realized because of technological or expense reasons. In this case, an overlapping of the oscillator frequency and the interference frequency occurs, which leads to undesired effects, particularly to beating, which makes it difficult or even impossible to analyze the receiving coil signals.
Therefore, it is an object of this invention to provide a cost effective and uncomplicated inductive angle sensor that is substantially insensitive to electrical, magnetic, or electromagnetic alternating fields having interference frequencies close to an operating frequency of an oscillator.